The long range goals of this work are the identification of Myosin heavy chain (MHC) isoforms and the factors which influence their appearance and accumulation during the development of chick skeletal muscle. Monoclonal antibodies (MCAb's) against adult chicken MHC have been used in radioimmunoassays, immunofluorescence and immunoblots to identify transitions in MHC isoforms during development. These antibodies and new MCAb's raised against embryonic pectoralis myosin will be used to affinity purify different MHC isoforms and to detect regions of homology and devergence between embryonic, neonatal and adult myosin. Transitions of the MHC will also be studies in cell culture preparations of embryonic skeletal muscle and compared to in vivo myogenesis. In addition, mRNA isolated from neonataland embryonic muscle will be translated in vitro and the translation products immunoabsorbed with various MCAb's. These experiments should distinguish between antigenic changes in the MHC which result from post-translational modifications of a single MHC gene product and the sequential expression of several MHC structural genes. This study will provide the background for subsequent nalysis of MHC gene expression during skeletal muscle regeneration and dystorphy. It will permit a direct attack on the question of whether muscular disorders, such as Duchenne Muscular Dystrophy, result from defects in the regulation of myosin gene switching during development.